Use the same sign for amount when splitting line by amount

Also increase interval by the interval value and set a currency to the
remaining term.

move.py

@@ -2208,20 +2208,22 @@ class SplitLines(Wizard):
             values['terms'] = terms = []
             if self.start.amount:
                 interval = self.start.interval
-                while abs(remaining - self.start.amount) > 0:
+                amount = self.start.amount.copy_sign(remaining)
+                while remaining - amount > 0:
                     terms.append({
                             'date': date,
-                            'amount': self.start.amount,
+                            'amount': amount,
                             'currency': currency.id,
                             })
                     date = (
                         self.start.start_date + relativedelta(months=interval))
-                    interval += 1
-                    remaining -= self.start.amount
+                    interval += self.start.interval
+                    remaining -= amount
                 if remaining:
                     terms.append({
                             'date': date,
                             'amount': remaining,
+                            'currency': currency.id,
                             })
             elif self.start.number:
                 amount = self.start.currency.round(

tests/scenario_move_line_split.rst

@@ -77,7 +77,30 @@ Create Lines to split::
     >>> receivable.balance, receivable.amount_second_currency
     (Decimal('90.00'), Decimal('100.00'))
 
-Split line::
+Split line by amount::
+
+    >>> split = Wizard('account.move.line.split', [line])
+    >>> split.form.total_amount
+    Decimal('100.00')
+    >>> split.form.start_date = period.end_date
+    >>> split.form.frequency = 'other'
+    >>> split.form.interval = 2
+    >>> split.form.amount = Decimal('30.00')
+    >>> split.execute('preview')
+
+    >>> term1, term2, term3, term4 = split.form.terms
+    >>> term1.date == period.end_date
+    True
+    >>> term2.date == period.end_date + relativedelta(months=2)
+    True
+    >>> term3.date == period.end_date + relativedelta(months=4)
+    True
+    >>> term4.date == period.end_date + relativedelta(months=6)
+    True
+    >>> term1.amount, term2.amount, term3.amount, term4.amount
+    (Decimal('30.00'), Decimal('30.00'), Decimal('30.00'), Decimal('10.00'))
+
+Split line by number::
 
     >>> split = Wizard('account.move.line.split', [line])
     >>> split.form.total_amount