Can anyone recommend an FCM that offers T-bill collateralization for trading and cost-effective T-bill position management? Thanks!

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This may not be a simple answer. Whether an FCM will allow you to use T-Bills as collateral, or even straight up pay you interest will depend on your level of activity and margin activity.

Unless you are planning on holding a Margined Position for a long period of time or have plenty of funds in Seg to cover said Margin position a T-Bill will only be beneficial with rates at 5% or higher. If you have a news event that takes your PnL (Loss) to a point where you need to break the T-Bill you may need to pay a penalty for early release of funds. Interactive Brokers have hinted at paying customers some small interest for deposited funds but there are a lot of rules attached to that. If you want to park cash for interest just go to: https://home.treasury.gov/services/bonds-and-securities

Ok so there is T-bill collateral. Depends on your broker on what they give you. TD and Schwab is like 4%-7% margin use and tasty is like 4% “on BIL etf”. The relief in margin can only be used on an index. SPY, SPX options, ES. So can be 90% range of margin give back. That’s about all I know. Not many brokers offer treasury auctions. Call them to see what the relief is from t-bills and treasury etfs like BIL and sgov