§ 13. Debts which may be proved against the estate. Debts of the assignor may be proved and allowed against his estate which are (a) a fixed liability, as evidenced by a judgment absolutely owing at the time of the assignment, or (b) a claim for taxable costs incurred before the assignment, in good faith, in an action to recover a provable debt; (c) or founded upon an open account, or upon a contract, express or implied whether due or not due; or (d) claims for anticipatory breach of contracts, executory in whole or in part, including unexpired leases of real or personal property; provided, however, that the claim of a landlord for damages for injury resulting from the rejection of an unexpired lease of real estate or for damages or indemnity under a covenant contained in such lease shall in no event be allowed in an amount exceeding the rent reserved by the lease without acceleration, for the year next succeeding the date of the surrender of the premises to the landlord or the date of reentry of the landlord, whichever first occurs, whether before or after the assignment, plus an amount equal to the unpaid rent accrued, without acceleration up to such date.

In allowing the claims against the estate, in all cases of mutual debts or credits between the estate of the assignor and a creditor the amount shall be stated and one debt shall be set off against the other, and the balance only shall be allowed.

A set-off or counterclaim shall not be allowed in favor of any debtor of the assignor which (a) is not provable against the estate; or (b) was purchased by or transferred to him after the filing of the general assignment or with intent to such use and with knowledge or notice, or if he had reasonable cause to believe, that such assignor was insolvent. A person shall be deemed insolvent whenever the aggregate of his property, exclusive of any property which he may have conveyed, transferred, concealed, removed, or permitted to be concealed or removed, with intent to defraud, hinder or delay his creditors, shall not, at a fair valuation, be sufficient in amount to pay his debts.