assalamu alaykum. suppose i gave some money to my friend as a loan. then he made so late to give it back. so i forgave the loan in my heart but i didn't inform him about that. after some days he wanted to give it back, in that case is it allowed for me to take it as i forgave it in my heart?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Since you had only intended to forgive your friend from having to repay you the loan, then it is permissible to get back the money that you had lent him and you are not sinful in doing so.

Allah knows best.