in the book the giver why would jonas running away give the memories the community he didn't die,?

They're not assuming that the memories would return, they know (It happened when Rosemary was released all of her memories of grief and depression were released into the community). When the Giver gives memories to Jonas, he loses them and only has a little "wisp" of what he gave to the Receiver. You need to remember that this is a utopia so it does not need to follow any solid logic, but I'm going to assume that when someone leaves the Communities(running away, dying), whatever is affecting them/messing with them (per se food, vaccinaitons, the air, other causes) starts to fade away so they release the memories they have from other people and become more human (THIS IS A THEORY), and when the memories fade form the carrier they go into the Communities where they are felt by the people who live there. In the end of the book Jonas finds human/modern civilization (possibly, or he and Gabe die) so you can also theorize that maybe the communities are an experiment. I believe that they didn't want to wait for the Five to grow up because it would take 7 years and they just didn't have that much time. The Giver and Jonas planned for the escape, and since they wanted a change to happen in the Community Jonas had to receive painful and moving memories like war, death, love, and family in order to cause a rebellion. If they had waited for the Five to grow up, Jonas would possibly release some memories that would discourage rebellion because people would be afraid and not want pain, which is why nothing happened when Rosemary was released. Her memories were all of grief and sadness. Many of Jonas's were happy and of Christmas and family. Since people take pills for the Stirrings, they don't feel happiness, love, pain, sadness, or anything else, just, indifference and contentment. If they felt feelings of love and pain, they would crave it and want to have more of it.

Hopefully I helped!