src/Model/Account.php
public function getCustomAssetBalanceStroops(Asset $asset)
    {
        MathSafety::require64Bit();

        foreach ($this->getBalances() as $balance) {
            if ($balance->getAssetCode() !== $asset->getAssetCode()) continue;
            if ($balance->getAssetIssuerAccountId() != $asset->getIssuer()->getAccountIdString()) continue;

            return $balance->getUnscaledBalance();
        }

        return null;
    }
public function getCustomAssetBalanceStroops(Asset $asset)
    {
        MathSafety::require64Bit();

        foreach ($this->getBalances() as $balance) {
            if ($balance->getAssetCode() !== $asset->getAssetCode()) continue;
            if ($balance->getAssetIssuerAccountId() != $asset->getIssuer()->getAccountIdString()) continue;

            return $balance->getUnscaledBalance();
        }

        return null;
    }
Returns the balance of a custom asset in stroops

@param Asset $asset
@return null|string
@throws \ErrorException