Custom resolvers for a flutter project allowing for incremental data updates (Amplify, DynamoDB)

I'm creating a POS and do not have back end experience. My Flutter app is using Amplify for most of its database requirements, however there is one element still remaining; providing a solution to a race condition -> using multiple devices to change the same object in the database.
This would usually involve creating a custom resolver for amplify in VTL, which is where i would like help.