What is the ruling of Islamic law regarding the borrower increasing the amount of the loan to the lender as a gift, without the lender's request, and is it permissible to add this increase to the check written to guarantee repayment?

Scholars agree that a lender stipulating an increase upon repayment of a loan invalidates the contract and constitutes prohibited usury (riba). However, if the increase or benefit is not stipulated, it is permissible according to the majority of scholars, based on the hadith (definition) of Abu Rafi’ that the Prophet (peace be upon him) repaid a camel superior to what he had borrowed, and said: "Indeed, the best of people are those who are best in fulfilling their obligations." Therefore, if the lender does not stipulate the increase, and the borrower voluntarily offers it, there is no harm in that.