Halfway was involved in the heist and helped himself .He may not have been aware it was so valuable .Then he used it to propose to Whitney which was a failure and left it on the table where Mick pocketed it .
As if anyone (even if they are only halfway) would let Mick keep the ring. He should have at least negotiated a share in the pub if he really wanted to help Mick out.
Anyway, with all the gang suspecting each other for stealing the money are they not all gonna be asking Mick where he got the money from and assuming he must have taken the heist money?