It’s a sin, but if he was a believer it is not the unforgivable sin, he can’t shed his salvation.
I think you can shed your salvation. After all, Annanias and Sapphira were believers and, in fact, far more committed than most believers these days. They believed enough to give half of their wealth to the church. Granted they lied about that amount, but that was enough to bring swift judgment and destruction.
Don’t be too quick to buy into this “once saved/always saved” theology, lest God give you up to your sins. Keep His commandments always as best you can, and when you fail, confess and undo, insofar as you are able, your sin.
Undoing your sin may cost you dearly, but if you do not you are not truly repentant, and yes, your salvation is indeed at stake. Do not think that because you are “saved” you can be a stumbling block for others, cause them to lose their faith, that then God will forgive and save you. He did not save Annanias and Sapphira who had become stumbling blocks. Do not think you are so cheaply bought that you can treat this gift of salvation lightly and keep it.
But if you are a repentant sinner, your salvation is not lost, and neither is this poor man’s if he kneels himself before our just but merciful God and asks forgiveness and the strength to undo this sin, though it may cost him his life.