What was the bargain that Callie made with her grandfather?

This is a really good question, and to answer it, we'll have to read Chapter 22 very carefully. The narrator, Callie, never tells us exactly what the bargain is although she does say that it's a "hard bargain" that she'd made with her grandfather. We have to infer what that bargain is.

We do know for sure that the situation at Thanksgiving is that Callie's brother Travis has become emotionally attached to the turkeys he's been taking care of. He doesn't want them to be killed for the holiday dinner. Callie's mother is nearly indifferent to Travis's emotional appeals, but Callie gets her to agree to trade the turkeys with another family's so that Travis won't have to witness his own creatures being served for the feast.

That seems like a good solution, but Callie has trouble executing it. No other families are ready to trade their turkeys. She asks Granddaddy for help, and at first it seems like he won't help, but then he asks her if it's important to her. She says that it's important to Travis, so by extension, yes, it's important to her, too.

But that's all that we see of their conversation. We assume that they worked out a bargain right then, but we don't get to read about what it was.

Here's the first clue we see after that:

"Next morning, looking down at the pen, you could see there were three new turkeys. They were a different color from ours, and they had fewer tail feathers..."

Those details are a little misleading. Callie seems to be telling us that the turkeys were different animals. But look closely and notice that all she's really saying is that the turkeys are a different color and that they don't have as many tail feathers.

Then, at the Thanksgiving dinner, we get our second clue:

"I bore some substantial scratches on my arms and... Granddaddy's nails were rimmed with crescents of dark paint."

So if Callie's arms are scratched, and if Granddaddy has paint on his fingernails, then the two of them had probably painted their own turkeys a different color and plucked out some of their feathers (explaining Callie's scratches) so that the tender-hearted Travis would see these turkeys and believe that they were not his own beloved animals.

That's how Granddaddy helped Callie. But she'd called their arrangement a bargain. What did she do in return for him? She volunteers to take care of the turkeys at the following Thanksgiving. (This happens in the final paragraph of Chapter 22.) Recall that Granddaddy had told Callie: "Our whole existence on this earth is a cycle of life and death. That is a fact. There is no stopping it." He probably wants Callie to take care of the turkeys herself the following year so that she can accept the sad truth about the brevity of lives, especially those of livestock.