What is the proper action regarding money in which there is a suspicion of unlawfulness?

The questioner did not specify the doubt of unlawfulness (haram) he has, but generally, the noble prophetic tradition: "The lawful is clear and the unlawful is clear, and between them are doubtful matters..." clarifies that avoiding doubtful matters absolves a person in his religion and honor. Doubts vary in their strength and weakness: some must be avoided, thus joining the unlawful; some are disliked (like an obsessive whisper); and some are recommended to be avoided, based on the Prophet's (peace be upon him) saying: "Leave what makes you doubt for what does not make you doubt," and this is for the sake of purification.

If the doubtful money is not forbidden to benefit from, but it is rather recommended to leave it, then there is no harm in benefiting from it based merely on a suspicion of doubt. Spending it on the needy or for public interests is better. Piety (wara') has levels: the first is abstaining from what the fatwa has forbidden (the piety of the upright), and its ultimate level is abstaining from everything that is not for Allah, whether acquired through desire or obtained by disliked means (the piety of the truthful ones - Siddiqeen). The more a servant is strict with himself, the lighter his burden will be on the Day of Resurrection. You have the choice between taking precautions or taking a concession.