Translation:I am selling a cat for four hundred and twenty-three euros.
I thought the genitive case was only mandatory if the last number was more than "čtyři".
... (něco) tři eur ? ... (něco) tři eura ?
It is not mandatory and it is accepted. It would be helpful to see your exact sentence.
Hmm. I thought the genitive plural was required for a "countable" noun that comes after any number above 4, which 423 is....
On the English side, in this sentence, "I sell A CAT for..." is not grammatically incorrect but it sounds unnatural. "I sell" would be fine in a sentence like "I sell CATS for..." or in a construction like "I sell A CAT every day for..."