Loved it. as per usual. I didn't think he was too casual at all. It probably works as a nice diversion for you too, keep thrashing little adventures like this out to keep your pen nib moist. Wish I could do it.
Don't have much to add in terms of the narrative as nothing seemed to jar.
But a few suggestions;
1. The retainer; really he should only have been paid a portion to take the job, the rest would come upon completion. That would also give him added incentive to see the job through.
2. A final footnote. Who was dipping in the tills? Did I miss something. Just a mention, even if Dave doesn't find out, to tie it up.
Good work that fella.