Combined Federal, state, and local bounties for a 3-year enlistment could also have amounted to several hundreds of dollars, which he would have received even if invalided out after only 3 months. Many soldiers arranged to have part of their pay sent home. Every northern state, I believe, also had some sort of "Family Aid" program, which provided payments to needy families of serving soldiers. It wasn't very much, but could make a huge difference. Families were rarely left to entirely " fend for themselves."
Unless he was a commissioned officer, he could not have "resigned," but must have been discharged for some reason: wound, illness, accident, or some other disability. If he went right home, he was likely seriously enough disabled not to be transferred to a Veterans Reserve Corps unit, but perhaps not enough to require long hospitalization.
Again, if you can provide a name, we can very likely find out much more.
jno