diff --git a/src/main/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipController.java b/src/main/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipController.java
index 62dcd1c2..de8cc01a 100644
--- a/src/main/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipController.java
+++ b/src/main/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipController.java
@@ -125,7 +125,7 @@ public class HsOfficeMembershipController implements HsOfficeMembershipsApi {
     }
 
     final BiConsumer<HsOfficeMembershipEntity, HsOfficeMembershipResource> SEPA_MANDATE_ENTITY_TO_RESOURCE_POSTMAPPER = (entity, resource) -> {
-        // TODO.refa: this should be possible via ModelMapper config
+        resource.setMemberNumber(entity.getTaggedMemberNumber());
         resource.setValidFrom(entity.getValidity().lower());
         if (entity.getValidity().hasUpperBound()) {
             resource.setValidTo(entity.getValidity().upper().minusDays(1));
diff --git a/src/main/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipEntity.java b/src/main/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipEntity.java
index a0740b8b..1b05a90c 100644
--- a/src/main/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipEntity.java
+++ b/src/main/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipEntity.java
@@ -130,6 +130,7 @@ public class HsOfficeMembershipEntity implements BaseEntity<HsOfficeMembershipEn
         }
         return validity;
     }
+
     public Integer getMemberNumber() {
         if (partner == null || partner.getPartnerNumber() == null || memberNumberSuffix == null ) {
             return null;
@@ -138,6 +139,10 @@ public class HsOfficeMembershipEntity implements BaseEntity<HsOfficeMembershipEn
         return getPartner().getPartnerNumber() * 100 + Integer.parseInt(memberNumberSuffix, 10);
     }
 
+    public String getTaggedMemberNumber() {
+        return MEMBER_NUMBER_TAG + getMemberNumber();
+    }
+
     @Override
     public String toString() {
         return stringify.apply(this);
diff --git a/src/main/resources/api-definition/hs-office/hs-office-membership-schemas.yaml b/src/main/resources/api-definition/hs-office/hs-office-membership-schemas.yaml
index 20459f4e..cc723a41 100644
--- a/src/main/resources/api-definition/hs-office/hs-office-membership-schemas.yaml
+++ b/src/main/resources/api-definition/hs-office/hs-office-membership-schemas.yaml
@@ -26,9 +26,10 @@ components:
                 mainDebitor:
                     $ref: 'hs-office-debitor-schemas.yaml#/components/schemas/HsOfficeDebitor'
                 memberNumber:
-                    type: integer
-                    minimum: 1000000
-                    maximum: 9999999
+                    type: string
+                    minLength: 9
+                    maxLength: 9
+                    pattern: 'M-[0-9]{7}'
                 memberNumberSuffix:
                     type: string
                     minLength: 2
diff --git a/src/test/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipControllerAcceptanceTest.java b/src/test/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipControllerAcceptanceTest.java
index 33073956..68320f21 100644
--- a/src/test/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipControllerAcceptanceTest.java
+++ b/src/test/java/net/hostsharing/hsadminng/hs/office/membership/HsOfficeMembershipControllerAcceptanceTest.java
@@ -78,7 +78,7 @@ class HsOfficeMembershipControllerAcceptanceTest extends ContextBasedTestWithCle
                       [
                           {
                               "partner": { "partnerNumber": "P-10001" },
-                              "memberNumber": 1000101,
+                              "memberNumber": "M-1000101",
                               "memberNumberSuffix": "01",
                               "validFrom": "2022-10-01",
                               "validTo": null,
@@ -86,7 +86,7 @@ class HsOfficeMembershipControllerAcceptanceTest extends ContextBasedTestWithCle
                           },
                           {
                               "partner": { "partnerNumber": "P-10002" },
-                              "memberNumber": 1000202,
+                              "memberNumber": "M-1000202",
                               "memberNumberSuffix": "02",
                               "validFrom": "2022-10-01",
                               "validTo": null,
@@ -94,7 +94,7 @@ class HsOfficeMembershipControllerAcceptanceTest extends ContextBasedTestWithCle
                           },
                           {
                               "partner": { "partnerNumber": "P-10003" },
-                              "memberNumber": 1000303,
+                              "memberNumber": "M-1000303",
                               "memberNumberSuffix": "03",
                               "validFrom": "2022-10-01",
                               "validTo": null,
@@ -125,7 +125,7 @@ class HsOfficeMembershipControllerAcceptanceTest extends ContextBasedTestWithCle
                       [
                           {
                               "partner": { "partnerNumber": "P-10001" },
-                              "memberNumber": 1000101,
+                              "memberNumber": "M-1000101",
                               "memberNumberSuffix": "01",
                               "validFrom": "2022-10-01",
                               "validTo": null,
@@ -153,7 +153,7 @@ class HsOfficeMembershipControllerAcceptanceTest extends ContextBasedTestWithCle
                       [
                           {
                               "partner": { "partnerNumber": "P-10002" },
-                              "memberNumber": 1000202,
+                              "memberNumber": "M-1000202",
                               "memberNumberSuffix": "02",
                               "validFrom": "2022-10-01",
                               "validTo": null,
@@ -196,7 +196,7 @@ class HsOfficeMembershipControllerAcceptanceTest extends ContextBasedTestWithCle
                         .contentType(ContentType.JSON)
                         .body("uuid", isUuidValid())
                         .body("partner.partnerNumber", is("P-10003"))
-                        .body("memberNumber", is(expectedMemberNumber))
+                        .body("memberNumber", is("M-" + expectedMemberNumber))
                         .body("memberNumberSuffix", is(givenMemberSuffix))
                         .body("validFrom", is("2022-10-13"))
                         .body("validTo", equalTo(null))
@@ -231,7 +231,7 @@ class HsOfficeMembershipControllerAcceptanceTest extends ContextBasedTestWithCle
                     .body("", lenientlyEquals("""
                     {
                          "partner": { "partnerNumber": "P-10001" },
-                         "memberNumber": 1000101,
+                         "memberNumber": "M-1000101",
                          "memberNumberSuffix": "01",
                          "validFrom": "2022-10-01",
                          "validTo": null,
@@ -256,7 +256,7 @@ class HsOfficeMembershipControllerAcceptanceTest extends ContextBasedTestWithCle
         }
 
         @Test
-        void parnerRelAgent_canGetRelatedMembership() {
+        void partnerRelAgent_canGetRelatedMembership() {
             context.define("superuser-alex@hostsharing.net");
             final var givenMembershipUuid = membershipRepo.findMembershipByMemberNumber(1000303).getUuid();
 
@@ -273,7 +273,7 @@ class HsOfficeMembershipControllerAcceptanceTest extends ContextBasedTestWithCle
                     .body("", lenientlyEquals("""
                     {
                          "partner": { "partnerNumber": "P-10003" },
-                         "memberNumber": 1000303,
+                         "memberNumber": "M-1000303",
                          "memberNumberSuffix": "03",
                          "validFrom": "2022-10-01",
                          "validTo": null,