Wolves and Lazio have an agreement regarding the fee (€7m) as well. However, the Premier League are looking to sign the player on loan with an option to buy. Lazio want that option to be obligatory.
Wallace is not a key player for Lazio and it makes no sense for them to hold on to him. The 24-year-old is better off joining a club where he will play more often.
A move to Wolves would be ideal for all parties and the Premier League side just need to provide Lazio with the right assurances now.