Summary
In the '19-'20 season, Melli (already 28 years old) was brought to New Orleans with the hope he could be a stretch big that would be effective playing next to Zion Williamson. However, the Pelicans found out that Zion was further behind on defense than expected, and playing those two together would lead to real defensive issues. At 6'9" and unathletic, he provides no rim protection on defense, so it's difficult to play him at center. He also isn't quick enough to defend on the perimeter. While that can be easy enough to hide in somewhere against second units, the lack of rim protection is the big problem. On top of that, Melli shot a below-average 34% from three in his rookie season, and was even worse this season at a ghastly 23%.

Melli was traded to Dallas as part of the J.J. Redick trade, and I do not expect the Mavs to have much interest in re-signing him this summer. Melli is already 30 years old, so I wouldn't be surprised if he had no real NBA offers this summer and ended up back overseas.
 
Cap Considerations
Melli will be a restricted free agent with a $5.1 million cap hold, $4.9 Qualifying Offer, and the Mavericks will have his Early Bird rights. However, the Mavericks may not tender him a QO, which would make him an unrestricted free agent. If the Mavericks re-sign Melli using the Early-Bird Exception, they could offer him a starting salary of up to 105% of the average salary this past season (roughly $10 million), which would be more than enough.

Since Melli will only have two years of service, he will be subject to the Gilbert Arenas provision. This means that any offer sheet he signs with a team besides Dallas can have a starting maximum salary of $9.5 million, but the third and potentially fourth years of that contract can have a large jump (up to the max). His cap hit would then be the average salary over the life of the contract.

If the Mavericks match, they could choose for his cap hit to either be his actual salary per season or the average over the life of the contract, provided the average fits within the Early Bird Exception. I do not expect this to come into play for Melli, as I think any offers for him would fit within the full Non-Taxpayer MLE. There's also a chance that Melli ends up settling for the minimum.