The leaving group is an H2AlO(-) and the reductant is AlH4(-) to make the intermediate. While I like that as my leaving group, I also like to write a mechanism that clearly shows how all of the hydrogens can be transferred. You would see that I have written my BH4(-) and AlH4(-) mechanisms to show the regeneration of tetravalent borate and aluminate species. However, as I said, the mechanism is basically correct and I like my mechanisms to explain everything or as much as possible.