I'm a British English speaker and the idiom only sounds natural to me as either:
fixed a time
fixed a place
Sometimes people swap out 'time' or 'place' for words that mean the same thing, such as:
fixed a date
fixed a location
But I've never heard 'fixed a meeting' and it took a while to realise what it meant. Gotube's comment has the most natural wording:
"They agreed to meet tomorrow to sign the contract, and fixed a time of 8 am"
Edit: Both the Cambridge and Collins dictionaries have 'to fix' listed as a verb meaning 'to arrange,' so maybe it's a regional thing. I'm just pointing out that I'm Northern and I'm not used to hearing it.
I also want to add that I've heard 'fix a price' before too, along with 'Can you fix it for me?' meaning 'Can you arrange it for me?' (except the last one sounds like London slang, whereas 'fix a time/date/price' is used throughout the UK).