You can return it whenever you like. I bought that book specifically for evangelizing.
Literal
When returned [permission-てもらっても] good [polite-ですよ]. That book [topic-は] evangelism-use [to-に] bought book [copula-ですから].
布教用 'for evangelizing / for spreading the faith' (布教 'proselytizing' + 用 'for use') — here used humorously for enthusiastically recommending a book. いつ返してもらってもいい 'it's fine whenever you return it' stacks the temporal いつ + permission てもいい + the benefactive もらう.