Choose the exact meaning of the idiom/phrase. He is the sort of man who would do anything for a consideration.

If he found some merit in it is the correct idiom/phrase.
The first option, 'out of love' can be expressed as doing something for love or for the person/thing you love. For instance, She baked the cake out of love for his birthday.
The second option is in sympathy with the aggrieved. Aggrieved is a person who is feeling resentment for being ill-treated. Hence, this phrase's meaning is being sympathetic for the person or situation which is badly treated.
The third option is If he found some merit in it. This set of words can be used in a sentence like, Youll be selected by him if he finds some merit in you.
The fourth option is if he were paid to do it. This phrase can be used in a sentence like, He would have done anything if he were paid to do it.
Now as we found that two options can be appropriate, but as per the syntax of the question sentence, we can set if he found some merit in it. Because, in both parts of the sentences, the voices are set in an active voice. So, we think that the option 3 is correct here.