Question Details:He is going to get 20% of the gross profit of the sale and then get another 25% of the net as a beneficiary; he will receive 40% of the money he buys it for and the property.
As a general rule, a trustee, as a fiduciary, is not allowed to self-deal (i.e. sell the trust assets to himself or his spouse at a profit), regardless of whether he is also a beneficiary of the trust. However, quite often a trust instrument allows the trustee to self-deal, especially if a trustee is a relative of the grantor of the trust. In this case, the trustee should be able to sell the trust assets to himself. You should read the trust instrument and see if self-dealing is allowed (read the section titled "Trustee's powers or the like).
Regardless of the language of the trust instrument, a trustee remains a fiduciary with respect to the trust beneficiaries and is not supposed to do anything contrary to the interests of the beneficiaries. A sale of the trust assets to himself is not necessarily harmful to the beneficiaries' interests, however, if for example he pays less than the fair market value, he is likely to be in violation of his fiduciary duties to the beneficiaries.
I suggest you consult an attorney knowledgable in trust administration in your area.