The Hollies' "Pay You Back with Interest"

This morning I listened to the first two discs of a Hollies' compilation album (I'm listening to the final disc as I write this).  I noticed some word play in "Pay You Back with Interest" that I'd always misunderstood.

The title and a recurring line have the phrase "Pay you back with interest."  I'd always assumed that this refers to the financial sort of payment and interest, as if the singer/speaker had a debt and had to repay the debt and the interest accrued.  But listening to it this morning, I realized that that's not really the situation (although since there are also the lines "It seems unfair to leave you / And sell myself the way I do," that other financially-focused parsing seems to be hinted at).  The interest is actually the denomination with which the singer/speaker is paying his debt.  It's an ablative of means where I'd always thought it was an ablative of accompaniment.

The song describes how the singer/speaker has been "travelin'," "wanderin'," and "movin'" and how he comes home only when "tired out."  He's been neglecting his girlfriend, and his paying back with interest is his promise to be more dedicated to her in the future.

That's probably pretty obvious, but it was enlightening to discover what the song's really about after not having really paid attention to the lyrics for years.