Fix transferable formula (#5956)

* Fix transferable formula

* nits

packages/api-derive/src/balances/all.ts

@@ -57,9 +57,14 @@ function calcShared (api: DeriveApi, bestNumber: BlockNumber, data: DeriveBalanc
   let transferable = null;
 
   if (data.frameSystemAccountInfo?.frozen) {
-    const frozenReserveDiff = data.frameSystemAccountInfo.frozen.sub(data.reservedBalance);
+    const { frameSystemAccountInfo, freeBalance, reservedBalance } = data;
 
-    transferable = api.registry.createType('Balance', allLocked ? 0 : data?.freeBalance.sub(bnMax(frozenReserveDiff, api.consts.balances.existentialDeposit)));
+    transferable = api.registry.createType(
+      'Balance',
+      allLocked
+        ? 0
+        : freeBalance.sub(bnMax(new BN(0), frameSystemAccountInfo.frozen.sub(reservedBalance)))
+    );
   }
 
   return objectSpread({}, data, {

packages/api-derive/src/balances/types.ts

@@ -41,7 +41,7 @@ export interface DeriveBalancesAllAccountData extends DeriveBalancesAccountData
    */
   lockedBreakdown: (PalletBalancesBalanceLock | BalanceLockTo212)[];
   /**
-   * Calculated transferable balance. This uses the formula: free - max(frozen - reserve, ed)
+   * Calculated transferable balance. This uses the formula: free - max(0, frozen - reserve)
    * This is only correct when the return type of `api.query.system.account` is `FrameSystemAccountInfo`.
    * Which is the most up to date calulcation for transferrable balances.
    *