Popularly but in all probability erroneously attributed to Martin Luther. Cf. this, from 1858 (undoubtedly far from the first such use of "purse"). My friend James Darlack, Director of the Goddard Library at Gordon-Conwell Theological Seminary, confirms that it is not supported in the 1980 source cited by Richard Foster on p. 250 of Money, sex and power (1985), in which (likely the book responsible for diffusing the error) it appears on p. 19 as follows:
There are three conversions necessary: the conversion of the heart, mind, and the purse.At my request Mr. Darlack also searched the electronic edition of the American edition of Luther's Works (as did several other ATLA librarians), but without success.