Getting the AssetClosingAmount for a transaction

You need to use the indexer API for that:

curl "https://algoindexer.algoexplorerapi.io/v2/transactions/G6RUAGPKXL2OTHCUJUSCGEP75P2YHEQLAJZR4OIM3R7N7YFFOIGQ?pretty"

{
  "current-round": 22790489,
  "transaction": {
    "asset-transfer-transaction": {
      "amount": 10000,
      "asset-id": 312769
    }
  }
}