J.C.C. Mays prints the following, under the title 'Latin Distich on Giving and Receiving', as an original poem by Coleridge:
Magna dabit qui magna potest: Mihi parva potenti,It's not, though. These lines are from a late 15th-century edition of Saint Ambrose's Praeit Epistola Nuncupatoria, and are the publisher's or editor's dedication to a man called Faustus Petro Coardo Mecoenatus.
Parvaque poscenti, parva dedisse sat est.
[Mays (ed) Poetical Works (2001) 2:898]
The last four lines, there, mean: 'to Fautus, a distich to be read on the offering of this little book: "he who has much may give handsomely; but for me, who has little, and who asks little, it is enough to have given little."'
I think it's unlikely that this is where Coleridge found the couplet (he wrote it in his Notebook (CB 3:4122), and later put it in letters and wrote it in books). I think it's more likely he found it in Cotton Mather, who quotes the lines in his Magnalia Christi Americana: Or the Ecclesiastical History of New England (1702):
We know that Coleridge read Mather's Magnalia in detail (Marginalia 3:816-26); although most of that has to do with Mather's account of witchcraft trials. I'd say, though, that he read it for more than just that. This also dates the 'poem': because Coleridge, not having a copy of his own, read (and annotated) Southey's copy of the Magnalia when he was staying with his old friend in 1812. I can also note that it gives me geniune, academic-nerdy pleasure, to be able to write 'Magnalia Marginalia'.