Done and dusted

Solution 1:

Done and dusted

documents were signed in times past using pens dipped in ink, drying time could be considerable as the application of the ink varied so much. To speed things up an absorbent dust was sprinkled on the fresh writing, left momentarily, and then tipped off the sheet, allowing the document to be rolled or folded there and then, and carried off. Thus, a deal was considered finalized when it was "done and dusted". done and dusted

It is used to say that something has been successfully completed.

I hope our agreement is done and dusted sooner than we expect.

Solution 2:

Done and dusted means something is completely finished.

The first citation in the OED is in the 20th century, long after the invention of the ballpoint pen.

1953 Brit. Bee Jrnl. 15 Oct. 669/1 All to be done and dusted before the National Honey Show. After this the grand clear up.

This lends doubt to the pen-and-ink and absorbent dust origin, especially as the quoted text from the other answer misses out the source's own doubt:

Anyone know the origin of the phrase 'done and dusted...?

I came across this one, but can't verify.