For a poem about money and the evil woes it has helped heap upon mankind, the tone of the poem is fairly light. It sort of resembles a whimsical retelling of some past event, kind of like when your grandparents say something like, "Back in my day we had to . . ."
Being written in the past tense immediately helps create this tone. If the poem were written in present tense, the tone might sound much more accusatory. The past tense makes the poem feel much more reflective.
The opening two lines especially help with the light, whimsical tone. The alliteration used in line 1 with "bane of blisse" and line 2's "fresh and fine" show the reader that Donne is having a little fun with the poem. He is writing about a very serious topic, but does it in a way that sounds like teasing and mocking instead of screaming and threatening.