Cross-border mergers and acquisitions (M&A) purchases are the value of deals involving the acquisition of a foreign company by a local company. A higher value indicates a higher degree of openness.
Cross-border mergers and acquisitions (M&A) sales are the value of deals involving the acquisition of a local company by a foreign company. A higher value indicates a higher degree of openness.
(引自：http://www.aric.adb.org/integration_indicators_technotes.php)
上面的意思是：
Cross-border mergers and acquisitions (M&A) purchases ：是在跨国并购交易中，本国公司并购外国公司的交易额。
Cross-border mergers and acquisitions (M&A) sales ：是指跨国并购交易中，外国公司并购本国公司的交易额。
至于为什么会有负数，请看这里：
Financial Flows: Net Inflows (sales - purchases) of Cross-Border Mergers and Acquisitions
Units: Millions of US dollars ($US) (引自：http://earthtrends.wri.org/text/ ... /variable-1400.html)
即跨国并购交易导致FDI直接投资的净流入=sales-purchases，即外国并购本国额减去本国并购外国额。
(因为外国并购本国，意味着外资流入；本国并购外国，意味着资本流出)
希望能对你有用。

一个国家(地区)的跨国交易净值，所以有可能是正的，表示资本净流入，或被并购的交易额大于本国并购他国企业的交易额，也可能是负的(资本净流出，或主动并购大于被并购额)