The writer , Stephen says that his friend , Todd had once borrowed a dollar from him and has still not re payed back.
Stephen tried many a times to somehow connect the topic to 1 dollar and make it easy for Todd to remember that the former had lent him money , but he fails to , as Todd just completely forgets about it..
in this time the writer goes through a painful thought that if Todd owes me a dollar and has forgotten it, then , there must be men to whom i owe a dollar and have forgotten it.....
thus it might be that :
we do lend money but don't forget that we have also borrowed money!!!
i guess this is what it is....i hope it helped you!!