Dead man's cañon / Lauran Paine.

Enhanced descriptions from Syndetics:

The Hightower Ranch cowboys discovered a mummified man and his horse in a desert cañon near Springville, Arizona, both shot in the head. Ordinarily that should have been the end of it, since many a man riding the outlaw trail up out of Mexico didn't make it through that god-forsaken country, but near the body the cowboys had dug up a box that contained five bloodstained packets of $100 bills, amounting to $10,000. Who kills a man and leaves behind $10,000? Sheriff Claude Rainey locked the money in the only steel safe in Apache County while he waited to see if anyone would lay claim to it.The only clue to the identity of the dead man that Sheriff Rainey can find is the shriveled up brand on the horse, which he sketches and sends to the Registrar of Brands. Later, two men arrive within two weeks of each other. The first, Fernando Bríon, informs Rainey that U.S. Marshal Jonas Gantt and his horse have been found shot in the head on his land across the border in Mexico, and leaves Gantt's personal effects with Rainey. The second man is Deputy U.S. Marshal Arch Clayton, who informs Rainey that the dead man found by the Hightower cowboys was his partner back in Raton, New Mexico. Both men arouse suspicion in Sheriff Rainey and add to the questions mounting in his mind about the identity of the bushwhacking killer - foremost among them whether he will show up in Springville, and what it is that he is really after?