Is this job offer legit?

I have been offered an admin job, I done an interview via zoom and it went well clearly even if I felt at the time it didn't.

This is the strange part, the salary is 40k a year they want me to sign a contract which involves a 30k upfront payment and I will receive the 10k at the end of the year. No weekly even monthly payment, uncommon right?

This is the part I dont like if im not up to standard after 3 months I lose the job and have to give the 30k back! If I don't there will be legal trouble.

I was happy to get a job offer finally but now after hearing this im nervous to accept, should I decline?