3/4 OD copper tubing used for water supply - need fitting

[Wilson Churchill]

I am remodeling a kitchen in one of my rentals. I cut one of the water shutoffs for the kitchen sink, because it has a slow leak. I bought both a push fitting and a compression shutoff, but both were just slightly too small for the tubing. The 5/8 od is too small, and the 7/8 is too large. The OD of the copper pipe is 3/4.. It looks like they used refrigeration tubing for the water supply.

I either need a shutoff that will fit directly onto 3/4 OD copper, or a fitting to convert from the 3/4 OD to the standard 5/8 OD for 1/2 copper plumbing. I'd rather not replace the whole service, because I would either have to cut out the floors or dig in the crawlspace to access the plumbing. Any ideas??