Is the borrower obligated to return the amount that its owner forgave him, after he had informed him of an untrue reason for borrowing?

As long as you borrowed with the intention of repaying and did not neglect to do so when able, there is no sin upon you. Allah will fulfill your debt as long as you are sincere in your intention to repay, even if you become unable or delayed. As for the amount (250), if the donor gifted it for a specific purpose, then it is not permissible for you to use it except for that purpose. You are obliged to return it to him when you are able, or to seek his forgiveness. As for the prayers of the creditors, as long as you are excused and given respite by the Shariah, no harm will befall you from them. And if they pray against you, the prayer of the oppressor is not answered.