straddle period - переходящий период
period liability - срок материальной ответственности, срок ответственности

как это совместить?

Одинаково непонятно и в английском, и в русском...

"Straddle Period Liability" means all Tax liabilities relating to the pre-Closing portion of any Straddle Period determined in accordance with Section 8.3.

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Straddle Period
(a) Income Taxes relating to the pre-Closing portion of any Straddle Period shall be determined and computed as if a fiscal year had been formally closed on the Closing Date, taking into account carry forward losses available for offset until such period and available tax credits, provided that:

(i) any Tax due in relation to the pre-Closing portion of any Straddle Period which is deductible from the basis of assessment of income Taxes will be treated as immediately deductible for the purpose of calculating income Taxes relating to the pre-Closing portion of any Straddle Period even if not immediately deductible under standard Tax rules; and
(ii) any allowance or tax credit which is calculated on an annual basis (including but not limited to research and development tax credit) shall be allocated between the period ending on the Closing Date and the period after the Closing Date in proportion to the number of days in each such period.

"переходящий период" не подходит, имхо