It is narrated from Abu Ishaq (may Allah have mercy on him) that a man was owed one thousand five hundred dirhams by another man. They refused to pay him until he waived the five hundred. Then he wrote the document for him and released the man (from the rest). Then he demanded the five hundred dirhams from him again, so the two of them took their case to Judge Shurayh (may Allah have mercy on him). The judge said to the witnesses: "Did he place the five hundred in his palm?" They said: No. So the judge gave the order, and it was returned to him. The Shaykh said: We too do not permit a reduction when it is made under a condition.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلح / 11349