What was the deal that Dill and Scout made in To Kill a Mockingbird?

One summer, Dill asked Scout to marry him.  She accepted his proposal.  He told her that she "was the only girl he would ever love."  She looked forward to their life together someday.

Many months later, during a conversation with cousin Francis, the topic of Dill came up.  Scout told Francis that she and Dill were "'gonna get married as soon as [they were] big enough.'"  She also explained to Francis that Dill had asked her to marry him the previous summer.  Scout mentioned that when she and Dill were married someday, she wanted to wait on him.  She did not like the idea of Dill learning how to cook and serve her, even though Francis thought that all men should know how to do these things.

When the next summer came, Dill did not arrive to Maycomb.  He usually spent his summers there.  Instead, he wrote to Scout and told her that "he would love [her] forever and not to worry, [because] he would come get [her] and marry [her] as soon as he got enough money together."