diff --git a/src/main/java/net/hostsharing/hsadminng/hs/office/debitor/HsOfficeDebitorController.java b/src/main/java/net/hostsharing/hsadminng/hs/office/debitor/HsOfficeDebitorController.java
index 32721f0d..86548373 100644
--- a/src/main/java/net/hostsharing/hsadminng/hs/office/debitor/HsOfficeDebitorController.java
+++ b/src/main/java/net/hostsharing/hsadminng/hs/office/debitor/HsOfficeDebitorController.java
@@ -24,6 +24,7 @@ import java.util.List;
 import java.util.UUID;
 
 import static net.hostsharing.hsadminng.hs.office.relation.HsOfficeRelationType.DEBITOR;
+import static net.hostsharing.hsadminng.repr.TaggedNumber.cropTag;
 
 @RestController
 
@@ -53,11 +54,11 @@ public class HsOfficeDebitorController implements HsOfficeDebitorsApi {
             final String currentSubject,
             final String assumedRoles,
             final String name,
-            final Integer debitorNumber) {
+            final String debitorNumber) {
         context.define(currentSubject, assumedRoles);
 
         final var entities = debitorNumber != null
-                ? debitorRepo.findDebitorByDebitorNumber(debitorNumber)
+                ? debitorRepo.findDebitorByDebitorNumber(cropTag("D-", debitorNumber))
                 : debitorRepo.findDebitorByOptionalNameLike(name);
 
         final var resources = mapper.mapList(entities, HsOfficeDebitorResource.class);
diff --git a/src/main/java/net/hostsharing/hsadminng/hs/office/debitor/HsOfficeDebitorRepository.java b/src/main/java/net/hostsharing/hsadminng/hs/office/debitor/HsOfficeDebitorRepository.java
index bb6cd0f2..b2b51df6 100644
--- a/src/main/java/net/hostsharing/hsadminng/hs/office/debitor/HsOfficeDebitorRepository.java
+++ b/src/main/java/net/hostsharing/hsadminng/hs/office/debitor/HsOfficeDebitorRepository.java
@@ -1,8 +1,10 @@
 package net.hostsharing.hsadminng.hs.office.debitor;
 
+import net.hostsharing.hsadminng.hs.office.membership.HsOfficeMembershipEntity;
 import org.springframework.data.jpa.repository.Query;
 import org.springframework.data.repository.Repository;
 
+import jakarta.validation.constraints.NotNull;
 import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
@@ -16,13 +18,16 @@ public interface HsOfficeDebitorRepository extends Repository<HsOfficeDebitorEnt
             JOIN HsOfficePartnerEntity partner
                     ON partner.partnerRel.holder = debitor.debitorRel.anchor
                         AND partner.partnerRel.type = 'PARTNER' AND debitor.debitorRel.type = 'DEBITOR'
-                WHERE cast(partner.partnerNumber as integer) = :partnerNumber
-                  AND cast(debitor.debitorNumberSuffix as integer) = :debitorNumberSuffix
-               """)
-     List<HsOfficeDebitorEntity> findDebitorByDebitorNumber(int partnerNumber, byte debitorNumberSuffix);
+                WHERE partner.partnerNumber = :partnerNumber
+                  AND debitor.debitorNumberSuffix = :debitorNumberSuffix
+            """)
+     List<HsOfficeDebitorEntity> findDebitorByDebitorNumber(int partnerNumber, String debitorNumberSuffix);
 
     default List<HsOfficeDebitorEntity> findDebitorByDebitorNumber(int debitorNumber) {
-        return  findDebitorByDebitorNumber( debitorNumber/100, (byte) (debitorNumber%100));
+        final var partnerNumber = debitorNumber / 100;
+        final String suffix = String.format("%02d", debitorNumber % 100);
+        final var result = findDebitorByDebitorNumber(partnerNumber, suffix);
+        return result;
     }
 
     @Query("""
diff --git a/src/main/java/net/hostsharing/hsadminng/hs/office/sepamandate/HsOfficeSepaMandateController.java b/src/main/java/net/hostsharing/hsadminng/hs/office/sepamandate/HsOfficeSepaMandateController.java
index 52ef2aad..976aad83 100644
--- a/src/main/java/net/hostsharing/hsadminng/hs/office/sepamandate/HsOfficeSepaMandateController.java
+++ b/src/main/java/net/hostsharing/hsadminng/hs/office/sepamandate/HsOfficeSepaMandateController.java
@@ -131,7 +131,7 @@ public class HsOfficeSepaMandateController implements HsOfficeSepaMandatesApi {
         if (entity.getValidity().hasUpperBound()) {
             resource.setValidTo(entity.getValidity().upper().minusDays(1));
         }
-        resource.getDebitor().setDebitorNumber(entity.getDebitor().getDebitorNumber());
+        resource.getDebitor().setDebitorNumber("D-" + entity.getDebitor().getDebitorNumber());
     };
 
     final BiConsumer<HsOfficeSepaMandateInsertResource, HsOfficeSepaMandateEntity> SEPA_MANDATE_RESOURCE_TO_ENTITY_POSTMAPPER = (resource, entity) -> {
diff --git a/src/main/resources/api-definition/hs-office/hs-office-debitor-schemas.yaml b/src/main/resources/api-definition/hs-office/hs-office-debitor-schemas.yaml
index bc3d402e..bca2f0a2 100644
--- a/src/main/resources/api-definition/hs-office/hs-office-debitor-schemas.yaml
+++ b/src/main/resources/api-definition/hs-office/hs-office-debitor-schemas.yaml
@@ -12,10 +12,10 @@ components:
                 debitorRel:
                     $ref: 'hs-office-relation-schemas.yaml#/components/schemas/HsOfficeRelation'
                 debitorNumber:
-                    type: integer
-                    format: int32
-                    minimum: 1000000
-                    maximum: 9999999
+                    type: string
+                    minLength: 9
+                    maxLength: 9
+                    pattern: 'D-[0-9]{7}'
                 debitorNumberSuffix:
                     type: string
                     pattern: '^[0-9][0-9]$'
diff --git a/src/main/resources/api-definition/hs-office/hs-office-debitors.yaml b/src/main/resources/api-definition/hs-office/hs-office-debitors.yaml
index 16554531..2e5c3383 100644
--- a/src/main/resources/api-definition/hs-office/hs-office-debitors.yaml
+++ b/src/main/resources/api-definition/hs-office/hs-office-debitors.yaml
@@ -17,7 +17,10 @@ get:
           in: query
           required: false
           schema:
-              type: integer
+              type: string
+              minLength: 9
+              maxLength: 9
+              pattern: 'D-[0-9]{7}'
           description: Debitor number of the requested debitor.
     responses:
         "200":
diff --git a/src/test/java/net/hostsharing/hsadminng/hs/office/scenarios/HsOfficeScenarioTests.java b/src/test/java/net/hostsharing/hsadminng/hs/office/scenarios/HsOfficeScenarioTests.java
index e5e47cf6..0a844632 100644
--- a/src/test/java/net/hostsharing/hsadminng/hs/office/scenarios/HsOfficeScenarioTests.java
+++ b/src/test/java/net/hostsharing/hsadminng/hs/office/scenarios/HsOfficeScenarioTests.java
@@ -238,7 +238,7 @@ class HsOfficeScenarioTests extends ScenarioTest {
     @Test
     @Order(2011)
     @Requires("Person: Test AG")
-    @Produces("Debitor: Billing GmbH")
+    @Produces("Debitor: D-3101000 - Test AG - main debitor")
     void shouldCreateExternalDebitorForPartner() {
         new CreateExternalDebitorForPartner(this)
                 .given("partnerPersonTradeName", "Test AG")
@@ -283,7 +283,7 @@ class HsOfficeScenarioTests extends ScenarioTest {
     void shouldCreateSepaMandateForDebitor() {
         new CreateSepaMandateForDebitor(this)
                 // existing debitor
-                .given("debitorNumber", "3101000")
+                .given("debitorNumber", "D-3101000")
 
                 // new sepa-mandate
                 .given("mandateReference", "Test AG - main debitor")