All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
When the creditor states, in any form, that he relieves the debtor from the debt, he has no right to change his mind and reclaim the debt that he had already waived. Moreover, there are conditions for the waiving of debt: the creditor is responsible, knows the exact amount of the debt, isn`t joking nor forced to waive the debt. If these conditions are met, the waiving of the debt is valid.
The Sharia maxim in this regard states, "What is waived can`t be restored, nor can a non-existent." "If a person waives a right he is permitted to waive, then it is waived and can`t be restored…..For example, if a creditor relieved a debtor from a debt and later decided to reclaim that debt, he isn`t permitted to restore it because the debtor is cleared from liability." {Durar al-Hukkam Fi Shareh Majalat al-Ahkam, V.1:45}.
In conclusion, if the situation is as stated in the above question, then the creditor isn`t permitted to reclaim the debt that he had relieved his uncle from. And Allah the Almighty knows best.