Heer buckle and another German items were found at large burned out dugout, near village Baburkin, Stalingrad pocket. The buckle was found with this helmet - link.
A lot of black ciphers from German shoulder straps were found nearby. Therefore, most likely the buckle belonged to a soldier from a sapper company.
The map where all the items were found and the video of the moment of discovery below.
— 10-15 USD by Russian Post
— 40-55 USD by Russian Post