Is it permissible to give charity with a debt amount owed by a man who died thirty years ago, when his heirs are troublesome people?

The debt you owe to the deceased is a trust that must be repaid to their heirs immediately, and it is not permissible to delay it. You must hasten to fulfill obligations before it is too late. Giving the debt away as charity does not absolve you, as the rightful owners are present. If you fear problems from the heirs, you can deliver it to them in a way that does not cause issues.