Typically, one rebalances positions as a percent of a portfolio using optimize.TargetWeights(weights)
or similar method. If the sum of the weights is less than 1 then that amount will remain as cash. To re-invest that amount into an ETF such as TLT simply set the weight of that ETF to be 1-sum(weights).
It might be easier to show with an example. If you wish, reply and attach a copy of your current algorithm.
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