Is it permissible for me to specify a percentage of the profit for a third partner in a sheep trade, knowing that the profit is variable?

There is no objection to taking commenda (mudarabah) funds from another friend, provided they are not mixed with the first commenda funds, because they are two independent contracts, and the loss of one cannot be offset by the profit of the other, unless the first capital provider permits mixing, or if it is customary practice, in which case it is permissible according to the preponderant opinion. In all cases, it is a condition that the share of both the agent (mudarib) and the capital provider in the profit be known.