Death Hunt

In the Yukon Territory in 1931, Albert Johnson (Charles Bronson), a solitary American trapper, comes across an organized dog fight. A white German Shepherd is badly injured and Johnson forcibly takes it, paying $200 to its owner, a vicious trapper named Hazel (Ed Lauter).
Aggrieved by his treatment and claiming the dog was stolen from him, Hazel leads several of his friends to Johnson's isolated cabin. Some begin shooting while others create a diversion. After the shooting of Sitka, the dog that Johnson has nursed back to health, the trapper kills one pursuer, Jimmy Tom (Denis Lacroix),
Once they discover that Johnson has bought 700 rounds of ammunition from the local trading post and paid in $100 bills, many conclude that he is the "mad trapper", a possibly mythical, psychopathic, serial killer who supposedly murders other trappers in the wilderness and takes their gold teeth. An old trapper, Bill Luce (Henry Beckman), warns Johnson that the law is coming for him. Johnson fortifies his cabin.
Sergeant Edgar Millen (Lee Marvin), commander of the local Royal Canadian Mounted Police post, seems a tough but humane man. He has a veteran tracker named "Sundog" Brown (Carl Weathers) and a young constable, Alvin Adams (Andrew Stevens), plus a new lover in Vanessa McBride (Angie Dickinson). He reluctantly agrees to investigate Hazel's accusations that Johnson stole his dog and murdered Jimmy Tom.
Millen leads a posse of mounties and trappers to the cabin. He parleys with Johnson, telling him that he has a pretty good idea of what happened and if Johnson comes with him they can get it sorted out. However, before Johnson can answer, one of the trappers opens fire. Several end up killed, including one who is shot by one of his own friends. The posse uses dynamite to blow up the cabin, but Johnson escapes, shooting dead a Mountie, Constable Hawkins (Jon Cedar).
Millen, Sundog and Adams, joined by Hazel with his tracker dogs, set off into the frozen wilderness after Johnson. The case has made front page news across the country, and many trappers join in the chase, attracted by the $1,000 bounty that has been placed on Johnson's life. Captain Hank Tucker (Scott Hylands), a Royal Canadian Air Force pilot, is sent by the government to join the hunt, which is causing a national embarrassment. He reveals that Johnson was a member of a United States Army special intelligence unit during World War I.
Johnson utilizes a number of tracking techniques to avoid Millen's posse and the bounty hunters, living off the land in treacherous winter conditions. As the hunt continues, Millen begins to respect Johnson's uncommon abilities, while growing to resent the intrusion of so many outsiders.
Luce comes across two of the trappers camping in the wilderness and shoots them both dead before pulling out their gold teeth. Luce, it seems, is the real mad trapper.
The pursuers catch up to Johnson. Tucker begins to strafe the area indiscriminately with his aircraft machine gun, killing Sundog. The enraged Millen and Adams shoot down the aircraft with their rifles; Tucker crashes into a canyon wall and is killed. Johnson escapes after killing Hazel.
Luce comes across Johnson and tries to kill him, presumably attracted by the reward. Johnson tricks him and captures him at gunpoint. Millen spots Johnson and opens fire; the bullet hits him in the face, rendering him unrecognizable. As they examine the body, both Millen and Adams spot the real Johnson, dressed in Luce's clothes, on a ridge above them. The man they shot was Luce dressed in Johnson's clothes.
The Mounties allow Johnson to flee into Alaska, well aware that everything he did was in self-defense. As the other pursuers appear, Adams tells them that Millen has killed Johnson. A trapper finds that the body has a pocket full of gold teeth, so they celebrate the killing of the "mad trapper".

Question: How did Johnson pay for the ammunition?
Answer: $100 bills