An allowance can let a bridge burn up to 0.25 renBTC after you approve that amount. It gives a named contract permission to use your tokens; it does not itself burn them or send Bitcoin.
What does an allowance authorize?
An allowance is a spending limit recorded by an ERC-20 token contract, the code that manages many tokens on Ethereum-compatible networks. You set it by calling approve, naming the contract that will handle your tokens and the amount it may use.
For a burn, the bridge contract may call burnFrom or move tokens before destroying them. The token contract checks that the bridge is the approved spender and that the allowance covers the amount. If either check fails, the transaction stops.
If you want the background on the return journey, how RenBridge turned locked BTC into renBTC describes the lock-and-mint side. That is separate from the token permission a contract may need when handling a burn.
For example, say you hold 0.30 renBTC and approve a bridge to use 0.25. It can use at most 0.25 under that approval; the remaining 0.05 stays yours. The allowance is tied to that token contract, spender address, and network.
How does approval lead to a burn?
A typical contract-based withdrawal takes these steps:
- Choose the wrapped token, destination chain, and receiving address. A wrapped token is a token on one chain that represents an asset held or issued through a bridge.
- Check the spender address shown by the app. It should match the verified contract for that bridge and network.
- Approve either the exact amount or a larger limit. This approval is an on-chain transaction and costs a network fee.
- Submit the burn or withdrawal request. The bridge uses the allowance, destroys the wrapped tokens, and records where the underlying asset should go.
RenVM was the system behind RenBridge’s wrapped-asset transfers. In the documented renBTC return flow, a user called the BTCGateway’s burn function directly and supplied a Bitcoin address. That differs from a flow where a separate app contract spends tokens through an allowance.
RenBridge is discontinued, and RenVM 1.0 shut down in December 2022. Treat old instructions as historical; an approval alone cannot make an inactive bridge release funds.
Which approval method should you choose?
An exact-amount approval limits what the spender can use if it is later misused, but you may need another approval for a later withdrawal. An unlimited approval saves that extra transaction, but leaves the contract permission to use more tokens than this one burn requires.
Some tokens support permit, a signed message that lets an app set an allowance as part of a later transaction. It can avoid a separate approval transaction, but only when the token and app both support it. The signature still grants permission, so check the spender and amount it covers.
Before signing, confirm the token balance, network, spender, allowance amount, and withdrawal address. If an approval already exists, check its remaining limit; it may be enough, or it may grant more access than you want. Some tokens also require an existing allowance to be set to zero before changing it to another nonzero amount.
I would use an exact allowance for a one-off burn, then revoke any leftover permission if the app will not be used again. The key choice is whether the burn contract needs permission to take your tokens—or whether the token’s own burn function acts directly on your balance.
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