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Generally speaking, a partnership interest is a capital asset, thus any sale of the interest could potentially give rise to capital gain or loss. However, tax partners who contemplate the sale of their equity interests should closely examine the underlying assets of the partnership. If the partnership is holding hot assets, i.e., certain assets that would give rise to ordinary income if sold by the partnership, the sale of the partner's interest in the partnership can give rise to ordinary income.
What would the character of gain of a sale in a partnership interest with the only asset being a contractual right to purchase property under a purchase and sale agreement? - It would really depend on what the underlying property is. Is it ordinary income property like inventory, or is it investment assets? If it were an investment asset then it would be capital and the sale of the partnership interest would give rise to capital gain or loss. I am 80% sure that the contractual right itself is not an ordinary asset and this will give rise to capital gain or loss. You would also have to consider the business purpose of the partnership. If it is a holding company it is pretty hard to say that it would have any ordinary assets if it does not actively manage anything...
Would the sale of the partnership be considered an "unrealized receivable" under Section 751? - No section 751 is referring to hot assets, as mentioned above. These are things like unrealized working company receivables or certain inventory items.
What you may have to be mindful of is IRC 704(c). This is basically saying that if you were not originally a partner when the partnership acquired the contractual right to purchase asset then you will most likely not receive an allocation of it's income or loss, whether or not it was sold on the date of the partnership disposition. 704(c) is kind of tricky but if you were a partner in the partnership on the date of acquiring the asset then there should be no issues.
IRC 704 - http://www.law.cornell.edu/uscode/text/26/704
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