Au Contra-ire.
Just because God has a predestined reward in heaven doesn’t mean we are performing a good work to make God a debtor.
There do exist people who are performing good works to gain favor as a debt, but they will lose what they have.
Charity implies one anticipates nothing in return for their gift.
A gift might fit as it implies nothing in return, otherwise it wouldn't be a gift, it would be an obligation.
Personally, I have to admit that I get something for my giving. Sometimes I give because it makes me feel good. Sometimes because I think I should. Sometimes I feel I'm paying back for the unexpected good done to me in the past. Some are more altruistic than others, but is it altruism? I get something from each.
Perhaps altruism is found in forgiving others for their trespasses against us?
As you can see, I have no answer, just more questions. Much in life seems to be like this the closer I look at it. Maybe I need glasses?