I would put the old one back on until the one you want arrives.

Seems like low miles for replacing a cassette and chainring. I have 20 year old bikes with 10's of 1000's of miles on them that have never had a chainring replaced. I get about 8-10,000 miles out of a cassette, usually only the "most used" cogs are worn. Chains I'll get 4-5K out of, although I have moved up to 10s on my commuter and will find out next season what kind of life I get from those.